Method for cleaning polysilicon

ABSTRACT

A method for cleaning polysilicon comprises steps of cleaning solid or granular polysilicon with an aqueous solution of dissolved ozone, and of cleaning with hydrofluoric acid the polysilicon receiving the above cleaning based on an aqueous solution of dissolved ozone, wherein the above steps are executed once in this order, or the above steps are repeated once or more in this order. Subsequent to the last cleaning step using hydrofluoric acid, a still other step of cleaning the polysilicon with pure water and then drying it is preferably added.  
     This method for cleaning polysilicon allows organic materials, particles and metal impurities adsorbed on the surface of polysilicon to be removed at a low cost, and to increase the freeing rate.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a method for cleaningpolysilicon (polycrystalline silicon) to serve as a material for themanufacture of silicon single crystals.

[0003] 2. Prior Art

[0004] Silicon single crystals for semiconductor devices have beenprincipally manufactured by the Czochralski method (to be referred to asCz method hereinafter). The Cz method consists of melting solid orgranular polysilicon in a quartz crucible in a furnace, immersing acrystal seed into the melt thus obtained, raising the crystal seedthereby allowing single silicon crystals to grow. Because polysilicon toserve as a material for the manufacture of single silicon crystals has ahigh reactivity, it is stored being helmetically sealed in a plastic bagjust until it is transferred into a quartz crucible. However, whenpolysilicon is placed in a bag for sealing, or removed from a bag foruse, it often reacts with oxygen in the air to form thereby an oxidefilm on its surface. The oxide film may have organic materials,particles or metal impurities adsorbed on its surface. If polysiliconwith such organic materials or the like adsorbed on its surface is melt,the growth of single silicon crystals is disturbed so that the freeingrate (the weight ratio of a single silicon crystal obtained from a unitweight of polysilicon) is problematically reduced.

[0005] To serve as a solution to the above problem, a method has beenknown in which polysilicon is cleaned with a mixed acid comprisinghydrofluoric acid and nitric acid just before the polysilicon istransferred into a crucible. With this method, however, organicmaterials and others dissolving in the mixed acid may easily re-adsorbon the surface of polysilicon. Thus, with this method it has beendifficult to remove organic materials. Moreover, the method requiressuch a large quantity of the mixed acid that the cost for cleaning isincreased.

SUMMARY OF THE INVENTION

[0006] The object of the present invention is to provide a method forcleaning polysilicon capable of removing organic materials, particlesand metal impurities adsorbed on the surface of polysilicon at a lowcost to thereby increase the freeing rate.

[0007] The first aspect of the present invention resides in a method forcleaning polysilicon which comprises the steps of cleaning solid orgranular polysilicon with an aqueous solution of dissolved ozone, and ofcleaning with hydrofluoric acid polysilicon just cleaned with the aboveaqueous solution of dissolved ozone, and includes executing, in theabove order, the above steps once, or repeating the above steps once ormore.

[0008] According to the first aspect of the present invention, thesurface of polysilicon is oxidized by cleaning polysilicon with anaqueous solution of dissolved ozone to thereby form an oxide filmthereon. The oxide film is removed when cleaned with hydrofluoric acid,and thus organic materials, particles and metal impurities which mightbe adsorbed on the oxide film will be removed together with the oxidefilm. Repeating the step will further enhance the removal of suchimpurities.

[0009] The second aspect of the present invention resides in a methodfor cleaning polysilicon based on the first aspect of the presentinvention which further comprises, following the last step of cleaningwith hydrofluoric acid, steps of cleaning polysilicon with pure water,and of drying it.

[0010] According to the second aspect of the present invention, additionof the step of cleaning polysilicon with pure water will further enhancethe effectiveness of the method with regard to the removal of organicmaterials, particles and metal impurities.

[0011] The third aspect of the present invention resides in a method forcleaning polysilicon based on the first or second aspect of the presentinvention wherein the concentration of ozone of the aqueous solution ofdissolved ozone is from 3 to 20 ppm, and the concentration ofhydrofluoric acid is from 0.1 to 5 wt. %.

[0012] According to the third aspect of the present invention, bysetting the concentration of ozone of the aqueous solution of dissolvedozone and the concentration of hydrofluoric acid in the above ranges,the effectiveness of the method with regard to the removal of organicmaterials, particles and metal impurities will be further enhanced.

[0013] The fourth aspect of the present invention resides in a methodfor cleaning polysilicon based on the first to third aspect of thepresent invention wherein the cleaning step using an aqueous solution ofdissolved ozone comprises, steps of jetting an aqueous solution ofdissolved ozone at solid or granular polysilicon placed in a chemicallyresistant basket, and of immersing the basket containing the polysiliconin an aqueous solution of dissolved ozone.

[0014] According to the fourth aspect of the present invention,combining the step of jetting an aqueous solution of dissolved ozone atpolysilicon and the step of immersing polysilicon in an aqueous solutionof dissolved ozone will further enhance the effectiveness of the methodwith regard to the removal of organic materials, particles and metalimpurities.

[0015] The fifth aspect of the present invention resides in a method forcleaning polysilicon based on the fourth aspect of the present inventionwherein the cleaning step using hydrofluoric acid comprises a step ofimmersing in hydrofluoric acid polysilicon, being contained in achemically resistant basket, which has been immersed in an aqueoussolution of dissolved ozone.

[0016] According to the fifth aspect of the present invention,introduction of the step using hydrofluoric acid will further enhancethe effectiveness of the method with regard to the removal of oxidefilm, organic materials, particles and metal impurities.

BRIEF DESCRIPTION OF THE DRAWING

[0017]FIG. 1 illustrates the steps of cleaning polysilicon according tothe present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0018] The aqueous solution of dissolved ozone used in this invention ishighly pure, has a high oxidizing activity even at a low concentration,and is readily available. The concentration of ozone of the aqueoussolution of dissolved ozone is preferably 3 ppm or higher. If it werebelow 3 ppm, its oxidizing activity towards the surface of polysiliconwould be insufficient, whereas if it were beyond 20 ppm, it would damagethe containers that might come into contact with the aqueous solution ofdissolved ozone. More preferably, the ozone concentration of the aqueoussolution of dissolved ozone is kept at 5 to 15 ppm. The hydrofluoricacid used in this invention has a concentration of 0.1 to 5 wt. %.Particularly preferably it has a concentration of 0.1 to 1 wt. %, ormore preferably 0.5 wt. %. If the concentration were below 0.1 wt. %, itwould not remove organic materials, particles and metal impuritiessatisfactorily. If the concentration were beyond 5 wt. %, it wouldundesirably raise the cost for cleaning.

[0019] An embodiment representing a method for cleaning polysiliconaccording to the present invention will be described with reference toFIG. 1.

[0020] As shown in step 11 of FIG. 1, solid or granular polysilicon isplaced in a chemically resistant basket. The material to be used for themanufacture of such a chemically resistant basket may include fluorineresins of polyethylene, polypropylene, polytetrafluoroethylene (Teflon),etc. Then, as shown in step 12, an aqueous solution of dissolved ozoneis jetted at the polysilicon contained in the basket. The jetting takesplace through a jet nozzle or shower. If an aqueous solution ofdissolved ozone is jetted at polysilicon contained in a basket which hasbeen placed in a vessel, then it will be possible to recover the aqueoussolution of dissolved ozone flowing out from the basket and reuse it asa part of the aqueous solution of dissolved ozone which will be used inthe next step 13. Then, as shown in step 13, the basket containingpolysilicon is immersed in an aqueous solution of dissolved ozone. Theimmersion takes place by transferring the basket in a first tank (notillustrated) filled with an aqueous solution of dissolved ozone.

[0021] Then, as shown in step 14, the basket containing polysilicon isimmersed in hydrofluoric acid. The immersion takes place by transferringthe basket in a second tank (not illustrated) filled with hydrofluoricacid. Hydrofluoric acid has filled the second tank so much as to allow,if the basket containing polysilicon is immersed in the tank, excesshydrofluoric acid to overflow the tank. The overflowed hydrofluoric acidis preferably returned with a pump to the second tank for circulation.Next, as shown in step 16, an aqueous solution of dissolved ozone isjetted at polysilicon contained in the basket removed from the secondtank. Next, as shown in step 17, the basket containing polysilicon isimmersed in hydrofluoric acid. The immersion takes place by transferringthe basket in a third tank (not illustrated) filled with hydrofluoricacid. Preferably for this process, as in step 14, hydrofluoric acid hasfilled the tank so much that, if the basket containing polysilicon isimmersed in the tank, excess hydrofluoric acid may overflow the thirdtank, and the overflowed hydrofluoric acid may be returned with a pumpto the third tank for circulation. Alternatively, a part of theoverflowed hydrofluoric acid may be returned with a pump to the secondtank used in step 14 for circulation. Next, as shown in step 18, thebasket containing polysilicon is immersed in pure water. The immersiontakes place by transferring the basket in a fourth tank (notillustrated) filled with pure water. For this process, pure water ispreferably heated to a specified temperature (for example, a temperaturebetween 25 and 60° C.). Next, as shown in step 19, polysilicon containedin the basket is dried. The drying takes place by a warm jet of air, orby vacuum drying in an atmosphere consisting of an inert gas such asargon. Lastly, as shown in step 21, dried polysilicon is removed fromthe basket.

[0022] Description has been given above with reference to the embodimentwhere polysilicon is cleaned while being kept in a chemically resistantbasket. However, polysilicon may be cleaned while being moved on achemically resistant conveyer, instead of being kept in a basket.

[0023] Next, an embodiment example of this invention will be describedwith a comparative example.

Embodiment Example 1

[0024] Solid polysilicon was treated according to the cleaning steps asdescribed in FIG. 1.

[0025] Firstly, solid polysilicon was transferred into a polyethylenebasket (step 11). The basket was placed in a tank A, and an aqueoussolution of ozone dissolved at 20 ppm was jetted through a nozzle atpolysilicon at a rate of 5000 cc/min for three minutes (step 12). Then,the basket containing polysilicon having received the jetting wasimmersed for five minutes in another tank B which had been filled withthe same aqueous solution of ozone having the same ozone concentration(step 13). Then, the basket was removed from tank B, transferred into astill other tank C which had been filled with 0.5 wt. % hydrofluoricacid, and kept there for five minutes (step 14). Then, the basket wasremoved from tank C, transferred into a still other tank D, and exposedto a jet of an aqueous solution of ozone dissolved at 20 ppm ejectedthrough a nozzle at a rate of 5000 cc/min for three minutes (step 16).Then, the basket was removed from tank D, transferred into a still othertank E which had been filled with 0.5 wt. % hydrofluoric acid, and keptthere for five minutes (step 17). Then, the basket was removed from tankE, transferred into a still other tank F which had been filled with purewater, and kept there for 15 minutes (step 18). Then, the basket wasremoved from tank F, and introduced into a warm air type heater to drypolysilicon (step 19). Lastly, dried polysilicon was removed from thebasket (step 21).

Comparative Example 1

[0026] Cleaning solid polysilicon was achieved by repeating practicallythe same steps as in Example 1, except that the cleaning steps using anaqueous solution of dissolved ozone were omitted. Specifically, steps11, 17, 18, 19 and 21 were practiced in this order.

Comparison and Evaluation

[0027] In Comparative example 1, oxide film with uneven thicknessesnaturally developed on the surface of polysilicon were removed withhydrofluoric acid alone, while in Embodiment example 1 application of anaqueous solution of dissolved ozone allowed the formation of an oxidefilm with a uniform thickness on the surface of polysilicon, which wasthen removed with hydrofluoric acid, and these steps were repeated.Thus, in Embodiment example 1, the etched amount was larger, and theformation of an oxide film on the surface of polysilicon and the removalof the oxide film from the surface of polysilicon could be achieved moreperfectly. A solid polysilicon piece cleaned by the method ofComparative example 1, and another solid polysilicon piece the same withthe above but cleaned by the method of Embodiment example 1 werecompared for their water repellency after being exposed to drops of purewater. The result showed that the solid polysilicon piece cleaned by themethod of Embodiment example 1 was superior in water repellency, assuggested by the above mechanisms underlying the two cleaning methods.

[0028] As described above, the method of this invention for cleaningpolysilicon comprising cleaning solid or granular polysilicon with anaqueous solution of dissolved ozone and then cleaning it withhydrofluoric acid, and executing the above steps once, or repeating thesteps once or more will make it possible to remove organic materials,particles and metal impurities adsorbed on the surface of polysilicon ata low cost, and to, as a result, increase the freeing rate.

What is claimed is:
 1. A method for cleaning polysilicon comprisingsteps of cleaning solid or granular polysilicon with an aqueous solutionof dissolved ozone, and of cleaning with hydrofluoric acid thepolysilicon which has undergone the cleaning step using the aqueoussolution of dissolved ozone, wherein the above steps are executed oncein this order, or the above steps are repeated once or more in thisorder.
 2. A method for cleaning polysilicon of claim 1 which furthercomprises, following the last cleaning step using hydrofluoric acid,steps of cleaning polysilicon with pure water and of drying said cleanedpolysilicon.
 3. A method for cleaning polysilicon of claim 1 or 2wherein the concentration of ozone of the aqueous solution of dissolvedozone is 3 to 20 ppm, and the concentration of hydrofluoric acid is 0.1to 5 wt. %.
 4. A method for cleaning polysilicon of anyone of claims 1through 3 wherein the cleaning step using an aqueous solution ofdissolved ozone comprises, steps of jetting an aqueous solution ofdissolved ozone at solid or granular polysilicon contained in achemically resistant basket, and of immersing the basket containing thepolysilicon in an aqueous solution of dissolved ozone.
 5. A method forcleaning polysilicon of claim 4 wherein the cleaning step usinghydrofluoric acid comprises a step of immersing polysilicon which iskept in a chemically resistant basket and has been cleaned with anaqueous solution of dissolved zone, in hydrofluoric acid.